1. Field of the Invention
The present invention relates to a heat treatment apparatus that irradiates a substrate, such as a semiconductor wafer and a glass substrate for a liquid crystal display device, with a flash of light, thereby heating the substrate.
2. Description of the Background Art
Conventionally, lamp annealers employing halogen lamps have been typically used in the step of activating ions in a semiconductor wafer after ion implantation. Such lamp annealers carry out the activation of ions in a semiconductor wafer by heating (or annealing) the semiconductor wafer to a temperature of, for example, about 1000° C. to about 1100° C. Such heat treatment apparatuses utilize the energy of light emitted from the halogen lamps to raise the temperature of a substrate at a rate of about hundreds of degrees per second.
In recent years, with the increasing degree of integration of semiconductor devices, it has been desired to provide a shallower junction as the gate length decreases. It has turned out, however, that even the execution of the process of activating ions in a semiconductor wafer by using the aforementioned lamp annealers which raise the temperature of a semiconductor wafer at a rate of about hundreds of degrees per second produces a phenomenon in which the ions such as boron and phosphorus implanted in the semiconductor wafer are deeply diffused by heat. The occurrence of such a phenomenon causes the depth of the junction to exceed a required level, which can be a hindrance to good device formation.
In view of this, techniques for irradiating the surface of a semiconductor wafer with a flash of light by using xenon flash lamps (hereinafter also referred to simply as “flash lamps”) to raise the temperature of only the surface of the ion-implanted semiconductor wafer in an extremely short time (several milliseconds or less) are proposed for example in U.S. Pat. Nos. 6,998,580 and 6,936,797. The flash lamps have a spectral distribution of radiation ranging from ultraviolet to near-infrared regions. The wavelength of light emitted from the flash lamps is shorter than that emitted from conventional halogen lamps, and it approximately coincides with a basic absorption band of a silicon semiconductor wafer. It is therefore possible to, when a semiconductor wafer is irradiated with a flash of light emitted from the flash lamps, rapidly raise the temperature of the semiconductor wafer with a small amount of light transmitted through the semiconductor wafer. It has also turned out that a flash of light emitted in an extremely short time of several milliseconds or less can achieve a selective temperature rise of only near the surface of the semiconductor wafer. Therefore, an extremely short-time temperature rise using the xenon flash lamps allows the execution of only ion activation with no deep ion diffusion.
As described above, the heat treatment apparatuses employing xenon flash lamps are annealers that are essentially suitable for heat treatment of shallow junctions, the need to, using xenon flash lamps, carry out ion activation of somewhat deeper junctions than ever has arisen in recent years. For activation of deeper junctions than ever, conceivable is a technique of increasing the duration of light emission from flash lamps more than ever, thereby to raise the temperature of not only the surface (a shallow portion) but also a deep portion of a semiconductor wafer by heat conduction. As a result, the ion activation of a deep portion of a semiconductor wafer below the surface, i.e., the activation of a deep junction, becomes possible.
However, increasing the duration of light emission from xenon flash lamps so as to raise the temperature of a deep portion increases the surface temperature of the semiconductor wafer more than necessary, thus undesirably resulting in the occurrence of wafer warpage due to the action of great thermal stress on the surface, or at worst, the occurrence of wafer cracking due to an abrupt thermal expansion.